The earth's climate varies naturally. Each component of this complex system evolves on a
different timescale. The atmosphere changes in hours, and its detailed behaviour is impossible to
predict beyond a few days. The upper layers of the oceans adjust in the course of a few seasons,
while changes in the deep oceans can take centuries. The animal and plant life of the biosphere
(which influences rainfall and temperature) normally varies over decades. The cryosphere (snow and
ice) is slower still: changes in thick ice sheets take centuries. The geosphere (the solid earth
itself) varies slowest of all - mountain-building and continental drift (which influence winds and
ocean currents) take place over millions of years.

Past natural climate changes offer vital insights into human-induced climate change. Studies
of past climates ("paleoclimatology") give a sense of the scale of future changes projected
by climate models. They also provide a crucial check on scientists' understanding of key climate
processes and their ability to model them.

Systematic global temperature records are available only since 1860. These include land-based
air temperature measurements and sea-surface temperature measurements. Such data need to be checked
carefully for any biases that may be introduced by changes in observation methods or sites. For
example, many meteorological stations have been located in or near cities. As cities grow, they can
have a significant warming effect on the local climate. Such effects must be – and are –
taken into account in estimating recent changes in global temperature.

Studies of earlier climates are based on indirect evidence. Changing lake levels, for example,
can reveal the past balance between rainfall and evaporation. Tree-rings, coral, ice-caps, or ocean
sediments can all preserve information about the past. Using a combination of measurements, models,
and "detective work", scientists convert the quantities they can measure (such as the
chemical composition of an ice-core sample) into the physical variables they wish to investigate
(such as the Antarctic temperature of 100,000 years ago).

The earth's climate has been dominated by ice ages for the past few million years. Ice
ages are almost certainly triggered by slow "wobbles" in the earth's axis and its orbit
around the sun. These wobbles affect the total amount of energy the planet receives from the sun and
in particular its geographic distribution. During an ice age, global temperatures fall by
5oC and ice-sheets advance over much of Europe and North America. Ice ages are separated
by warmer "interglacial" periods.

Changes in greenhouse gas concentrations may have helped to amplify ice-age cycles. The small
fluctuations in energy arriving from the sun due to the earth's orbital wobbles are not large
enough to account for the size of global temperature changes during the ice age cycles. Ice-core
samples show that greenhouse gas levels also varied significantly and may have played an important
role in amplifying temperature fluctuations.

Reconstructions of past climates can be used as a check on climate model projections.
Comparing a model "prediction" of ice-age climate with the evidence from paleoclimatology
provides a crucial check on the model's representation of processes relevant for future climate
change. But the paleoclimatic evidence can be ambiguous: some sources suggest that, compared with
today, tropical seas were some 5oC colder at the peak of the last ice age, while others
suggest only 1-2oC. As a result, separating model errors from uncertainties in the
evidence can be difficult.

The climate seems to have been remarkably stable since the last ice age ended 10,000 years
ago. As far as scientists can tell, global temperatures have varied by less than one degree since
the dawn of human civilisation. Against the apparently extreme and sometimes rapid climate
fluctuations of the preceding 100,000 years, this stands out as a relatively peaceful interglacial
period.

Models predict that the climate could be warmer by the end of the 21st century than it was during
any previous inter-glacial period. In a period between two ice ages about 125,000 years ago, much
of Europe and Asia appear to have been about 2oC warmer than they are today. However,
models are predicting that temperatures could rise by much more than this over large stretches of
this region during the 21st century if greenhouse gas emissions continue as projected.

Abrupt climate variations in the distant past appear to have been traumatic for life on earth.
The earth's biological history is punctuated by so-called "mass extinction events"
during which a large fraction of the world's species are wiped out. There are many possible
reasons for mass extinctions, but the records suggest that some of these events coincided with
relatively abrupt changes in climate – similar in magnitude to the kind of change now forecast
for the 21st century. Over the next 100 years we may experience conditions unknown since before the
ice ages began many millions of years ago.